1. Field of the Invention
The present invention is directed to a method for printing an image in two colors.
2. Description of the Related Art
Such print images are also called highlight color images or, in short form, HLC images. The printing thereby ensues according to image data such that each image datum describes a color value of a picture element of a master image, and the color values of specific standard color regions are allocated to a standard color to be printed, and the color values of specific HLC color regions are allocated to an HLC color to be printed. The standard color and the HLC color are printed according to the color saturation of the colors of the master image allocated to the standard color or, respectively, to the HLC color.
In highlight color printing, a standard color and an HLC color are printed in different brightness levels or, respectively, color saturation values. As a rule, the standard color is black and is printed in different gray levels.
Highlight color printing is significantly more cost-beneficial than full-color printing with four primary colors (which is referred to as four-color printing), whereby the greatest variety of colors can be achieved by superimposed printing of the primary colors cyan, magenta, yellow and black. What is disadvantageous about four-color printing is that the luminosity of the colors that can be printed is limited despite the variety of colors. Particularly in the areas of red, green and blue, a luminosity level required by a client and especially a customized special color can often not be achieved in four-color printing.
Since only a specific color is often desired, it is significantly better for such applications to employ specific inks that exhibit the desired color when they are printed on widely distributed areas of the paper.
A known method for printing and HLC image comprises the following steps:    a) The master image is described by image data, whereby each image datum indicates the color value of a picture element. These color values are usually composed of three individual values for the colors red, green and blue. Each individual value is represented by a byte. Groups of similar color values are allocated to a reduced color value that is represented by a single byte. As a result thereof, the quantity of data of the image data of a master image is reduced to one-third.    b) The master image described with the reduced color values is converted into a print master image for the standard color that contains the color saturation values of the standard color for each picture element. These color saturation values are usually represented by a respective byte. Given the standard color black, the color saturation values represent the gray level. In the determination of the print master image, each picture element of the master image is examined to see whether the reduced color value thereof lies in the region of a standard color region that is allocated to the standard color to be printed and, when this is the case, the corresponding color saturation value is determined for this reduced color value and is stored in the print master image. The determination of the color saturation value usually ensues with pallets, i.e. tables, wherein the corresponding color saturation values are respectively allocated to the reduced color values.    c) The master image described with the reduced color values is converted into a print master image for the HLC color that contains the color saturation values of the HLC for each picture element. These color saturation values are usually represented by one byte. In the determination of the print master image, however, each picture element of the master image is investigated to see whether the reduced color value thereof lies in the range of a HLC color region that is allocated to the HLC color to be printed, and, when this is the case, the corresponding color saturation value is determined for this reduced color value and is stored in the print master image. The determination of the color saturation value likewise ensues with pallets as in the case of the standard color.    d) Since digital printers can usually only print color in two degrees of color saturation (no color or, respectively, full color saturation) or only in a very few color saturation levels, the master print image for the standard color and the master print image for the HLC color is converted into a rastered image with a dither method, this raster image being described with half-tone values.
The publication “Das Druckerbuch, Technik und Technologien der OPS-Hochleistungsdrucker, Drucktechnologien,” Addition 4a, May 1999, ISBN 3-00-001019-X, Chapter 2.2 yields a dithering method wherein an image is scanned point-by-point with the resolution to be printed and is compared to the thresholds of what is referred to a dither matrix. When the color saturation values of the scanned image are higher than the thresholds, then the half-tone value 1 is set at the corresponding location in the master print image. When the color saturation value is lower than the threshold, then the half-tone value 0 is set in the master print image. At the printer, the color to be printed is printed only at the locations or, respectively, output pixels whose half-tone value is equal to 1.
With this known dithering method, master print images can be generated for digital printers that resolve a master image into a raster that, due to the limited resolution capabilities of the eye, appears to a viewer of the print master image as an image having different color saturation regions, even though the individual printing points have either full color saturation or no color saturation. Such a color saturation level generated by a raster is referred to as half-tone.
What is referred to as a multi-level dither method has been developed for printers that can print the individual output pixels in different color saturation levels, this being disclosed, for example, by German Patent Document DE 40 25 793 C2. A similar method is disclosed in the publication “Multi-Level Halftoning, Paul A. Delabastita, Agfa-Gevaert N. V., IS&T-S Fourth Technical Symposium on Prepress, Proofing & Printing (1995)”, pages 68–73. These multi-level dithering methods are utilized given printers that can print an output pixel in a plurality of brightness levels. Given the exemplary embodiment disclosed by the German Patent Document DE 40 25 793 C2, the printer can print in six half-tone levels (see FIG. 5 of DE 40 25 793 C2). Accordingly, a master image described with color values is converted into a print master image described by print values, whereby the individual print values can assume different values (for example, six different values) that correspond to the brightness of the output pixels to be printed. This number of printing values cannot be generated with a single dither matrix; rather, a table having half-tone printing reference patterns is employed, whereby each half-tone printing reference pattern contains the print values of a printing raster that, when printed, represents a specific half-tone or, respectively, brightness value. In the known method, a half-tone printing reference pattern whose half-tone value corresponds to the color value of the printing point is selected based on the measure of the color value of the individual printing point. Printing values, i.e. the value that indicates the intensity with which an output pixel is to be printed with the printer, are arranged in a two-dimensional raster in the half-tone printing reference pattern, whereby each raster point is allocated to a specific picture element. The print value of the raster point corresponding to the picture element is taken from the half-tone printing reference pattern and is stored at the location of the respective picture element in the master print image. In this way, a master print image can be generated wherein a plurality of brightness levels of the individual color values are converted with a half-tone raster onto output pixels having a few brightness levels. The table with the half-tone printing reference patterns is employed instead of the dither matrix when the output pixels are to be printed with different brightness levels (multi-level), since the color value or, respectively, brightness value of a master image can only be converted with the known dither matrix onto digital output pixels (by-level) that have either full color saturation or no color saturation.
These known dithering methods have hitherto been utilized only for rastering a quasi-analog master image into a bi-level or, respectively, multi-level master print image.
European Patent Document EP-A2-0 781 035 discloses a method for producing highlight color copies from a multi-color original. In general, known method are employed for generating the HLC image. One special characteristic of the method is that the master image is automatically segmented, whereby different imaging methods can be employed for imaging the individual segments onto the HLC image.
U.S. Pat. No. 5,237,517 is directed to a method for imaging a color image onto a HLC image, whereby the color space of the HLC image is represented by a planar, triangular color space. The three corners of this color space represent the colors white, black and the fully saturated HLC color. In order to be able to image a specific hue of the chromatic master image onto the HLC image, that point in the color space of the HLC color is determined that corresponds to the fully saturated hue of the hue to be imaged. Together with the points for the colors black and white, this point in turn establishes a triangular sub-color space that represents a sub-set of the color space of the HLC color. The corresponding saturation levels of this hue are imaged onto this sub-color space.
U.S. Pat. No. 5,862,305 discloses a method for converting a plurality of input rasterings into a common output rastering. This method is particularly suited for highlight color systems, whereby the highlight color data for each color plane are separately processed and are subsequently merged.
U.S. Pat. No. 5,982,924 discloses a method for imaging a color image onto a two color image that allows the user to select the two colors onto which a color image is to be imaged.
German Patent Document DE-A1-195 47 200 discloses a method for generating an image with an accent color. The scanned master image is thereby examined with respect to bright or unsaturated color regions or saturated color regions. The saturated color regions are represented with a dark or saturated color and the bright color regions are represented with a second, bright color.